Ink-jet printers capable of forming color images using an ink set derived from cyan, yellow, magenta, and black inks are now commercially available. Specific types of ink-jet printers, such as thermal and piezo-electric, are also well-known.
The formation of color images onto the print medium, e.g., paper, requires inks having competing requirements. For example, the ink must be relatively quick drying, so as to avoid smearing of images. Yet, the ink must not be so quick drying that it clogs the printhead nozzles out of which it is jetted. Further, when printing color images, bleed (the invasion of one color by another) must be avoided. Various patents have been issued dealing with dye-based color components; examples of such patents, which are assigned to the same assignee as the present application, include U.S. Pat. Nos. 5,091,005, 5,098,476, 5,100,470, 5,106,416, 5,108,503, 5,112,399, 5,116,409, 5,118,350, 5,133,803, 5,196,056, 5,198,023. Typically, these inks comprise a vehicle (one or more water-miscible organic solvents and water) and one or more water-soluble dyes, or colorants. Additives may be present to improve a given property, such as waterfastness, color bleed, and the like.
Penetration of the ink into the print medium (e.g., paper) reduces bleed, while cost per copy is reduced by lateral spreading of the ink. Cockle of the paper is reduced by a lower drop volume (e.g., less water). On the other hand, edge acuity is reduced for an ink that has a high lateral spread on the paper.
Further tradeoffs are realized by considerations of surface tension and viscosity. The surface tension governs the wetting properties of the ink to the paper, and this in turn is governed by the nature of the paper (e.g., photocopy versus bond). Viscosity controls the amount of ink dot spread, with a higher viscosity limiting dot spread.
Dot gain is defined herein as the amount of spread for a given dot size compared to no spreading. Thus, a low dot gain ink does not spread as much as a high dot gain ink. Another way of viewing this is that for a given dot size, where a smaller drop volume give the same dot size as a larger volume of ink, then gain is attained.
A print cartridge with a high dot gain ink is advantageous in reducing cost-per-copy, bleed, and paper cockle. However, the best edge acuity of a high dot gain ink is inferior to the best edge acuity of a low dot gain ink. When printing black text, edge acuity is the most important feature. The amount of ink used in a single black text page is usually minimal. However, edge acuity is less important when printing color text because the contrast between the color print and white page is less than the contrast between the black print and the white paper. Furthermore, bleed, cockle, and cost-per-copy are more important in a color page as large amounts of inks are printed. Thus, a printer with a print cartridge containing a low dot gain black ink and at least one other print cartridge containing a high dot gain color ink would be advantageous.
It will be appreciated by those skilled in this art that reducing the amount of ink used to jet a given droplet of ink while maintaining high edge acuity for text printing requires balancing various considerations. Thus, an ink set that combines the advantages of both low dot spread and high dot spread, while alleviating the disadvantages of each, is required.